Thermoplastic elastomers exhibit rubber elasticity at room temperature and are easy to mold since they are readily plasticized and melted upon heating. Thermoplastic elastomers are also recyclable. These features have made the thermoplastic elastomers widely used in a range of applications, including automobile parts, parts of home electric appliances, construction materials, sports equipment and articles of daily use.
Among different thermoplastic elastomers, styrene-based thermoplastic elastomers, such as polystyrene-polybutadiene-polystyrene (SBS) block copolymers, polystyrene-polyisoprene-polystyrene (SIS) block copolymers and hydrogenated products of these block copolymers, are particularly widely used since they are inexpensive, have high flexibility and rubber elasticity, and are readily recyclable.
Several proposals have been made to improve different physical properties of styrene-based thermoplastic elastomers. One example is a thermoplastic elastomer resin composition (1) for use in powder molding (See, Japanese Patent Application Laid-Open No. 2001-158812). This composition is intended for use in the production of molded articles with soft texture and high scratch resistance and contains a styrene-based thermoplastic elastomer and a polyurethane-based thermoplastic elastomer, among other components. Another proposal is a thermoplastic elastomer composition (2) (See, Japanese Patent Application Laid-Open No. Hei 5-230322). This composition offers high flexibility, moldability and scratch resistance and is formed by adding, to a composition composed of a styrene-based thermoplastic elastomer (hydrogenated block copolymer) and an acrylic resin, a copolymer including units compatible to each of the two components.
Also, several thermoplastic resin compositions have been proposed that retain surface properties, such as surface hardness, weather resistance, as well as transparency and other characteristics of acrylic resins and at the same time exhibit flexibility and low temperature properties. One example of such thermoplastic resin compositions is an acrylic thermoplastic resin composition (3) (See, Japanese Patent Application Laid-Open No. Hei 6-329865). This composition contains a hydrogenated product of a block copolymer and an acrylic resin with a particular intrinsic viscosity in predetermined proportions. The block copolymer contains polymer blocks of a particular molecular weight formed of a vinyl aromatic compound and polymer blocks composed of isoprene or isoprene and butadiene. Another example is a thermoplastic resin composition (4) (See, Japanese Patent Application Laid-Open No. Hei 5-295216). This composition contains, in predetermined proportions, an acrylic resin and a hydrogenated product of a triblock copolymer that has a particular number average molecular weight and containing blocks of an aromatic vinyl monomer and blocks of isoprene and/or butadiene. A thermoplastic resin composition (5) has also been proposed that exhibits high flexibility and weather resistance and offers a favorable appearance (See, Japanese Patent Application Laid-Open No. Hei 5-345841). This composition contains (i) a polyolefin resin; (ii) a hydrogenated product of a thermoplastic block copolymer composed of blocks of aromatic vinyl monomer and blocks of isoprene and/or butadiene; (iii) an acrylic resin; (iv) a hydrocarbon-based softener; and (v) a hydrogenated product of a thermoplastic block copolymer having a polymer side chain of acrylic monomer and composed of blocks of aromatic vinyl monomer and block of isoprene and/or butadiene.
While the composition (1) has an improved scratch resistance, though not as high as that of the polyurethane-based thermoplastic elastomer, the poor hydrolysis resistance and poor weather resistance of the composition make articles molded from the composition susceptible to performance deterioration and yellowing. The compositions (2) through (5) each retain surface properties, such as surface hardness, as well as weather resistance, transparency, and other characteristics of acrylic resins, while exhibiting high flexibility, high moldability, and high transparency. Nonetheless, each has its own drawback. Specifically, the composition (2) was tested for the scratch resistance according to JIS Z 8741 by rubbing 100 times with a No. 3 cotton cloth while a 500 g load was applied. The results of the test were given as the difference in the gloss of the sample before and after the test. The composition (5) was subjected to the pencil scratch test according to JIS K 5400. These tests demonstrated that neither of the compositions had sufficient scratch resistance nor abrasion resistance. As far as the abrasion resistance of the compositions (3) and (4) is concerned, nothing was mentioned in the respective publications. Thus, a demand exists for a thermoplastic polymer composition that is suitable for use in applications where it is subjected to frequent friction and aesthetic appearance is important.
Accordingly, it is an object of the present invention to provide a polymer composition that has high moldability, flexibility, rubber elasticity, mechanical properties, and transparency and at the same time exhibits scratch resistance and abrasion resistance comparable to polyurethane-based thermoplastic elastomers and polyester-based thermoplastic elastomers.
The present inventors have devoted significant effort to finding a way to achieve the above-described object and, as a result, have made a finding that by adjusting the proportions of components of a polymer composition that comprises an addition polymerization-based block copolymer, an acrylic resin and, if necessary, a softener, the addition polymerization-based block copolymer forms a continuous phase (matrix) with the acrylic resin dispersed throughout it, forming an sea-island structure (morphology). The addition polymerization-based block copolymer is such that it has a hard segment that is a polymer block A that essentially comprises an aromatic vinyl compound unit containing at least 1% by mass of an alkylstyrene-derived structural unit (I) in which at least one alkyl group having 1 to 8 carbon atoms is bound to a benzene ring.
The present inventors examined the physical properties of the polymer composition and articles molded from the polymer composition and found that the polymer composition is highly moldable and has many advantageous properties, including flexibility, rubber elasticity, mechanical properties, transparency, and, in particular, scratch resistance and abrasion resistance. The polymer composition of the present invention has these properties in a well-balanced manner, which makes the polymer composition suitable for use in various applications. This finding led the present inventors to devise the present invention.